Conventional refrigerator vehicles for overland transport of goods such as frozen foods while keeping those at low temperatures have been known. In such a refrigerator vehicle, for example, a refrigeration system for vehicle as disclosed in Patent Document 1 is provided. This refrigeration system includes a sub-engine separately from an engine for driving the refrigerator vehicle, and an electrically-driven compressor driven by the sub-engine. The number of revolutions of this electrically-driven compressor is controlled by an inverter circuit.